Sensors such as reduction sensors or contact sensors are available as image sensors used in image readers such as image scanners and facsimile apparatuses. Of these sensors, a contact image sensor (to be abbreviated as a CIS hereinafter) comprises a lighting apparatus, a one-to-one image formation optical device, a line sensor, and the like. Such a CIS generally has a shorter optical path length than an image sensor using a reduction optical system, and hence allows easy reduction in the size of a device which incorporates the sensor. For this characteristic, such CISs have been often used in low-profile flatbed type image readers and the like in place of reduction optical systems. A linear lighting apparatus used in such a CIS is required to illuminate a document surface at a necessary illuminance and allow the light reflected by the document to reach the line sensor with a sufficient intensity.
FIG. 12 depicts a sectional view explaining the arrangement of a conventional contact image sensor. In this case, the sensor includes a single optical waveguide member 2. This contact image sensor includes a light source unit 1 for applying light to a document 9 and applies light from the light source unit 1 onto the document 9 via the optical waveguide member 2. The light applied in this manner is reflected by the document 9 and strikes a line sensor 5 comprising photoelectric conversion elements via a lens array 4. This light is converted into an electrical signal.
Referring to FIG. 12, reference numeral 3 denotes a frame which supports the constituent members. The line sensor 5 has light-receiving units in the form of a plurality of lines, which photoelectrically convert an optical image of the document into an electrical signal. Reference numeral 6 denotes a sensor board on which the line sensor 5 is mounted. Reference numerals 1-r, 1-g, and 1-b respectively denote LEDs for applying light to the document 9 in the light source unit 1. These LEDs respectively emit red, green, and blue light beams. The LEDS are arranged on an end face of the optical waveguide member 2 which extends in the longitudinal direction. The optical waveguide member 2 is designed to receive light emitted from the respective LEDs and make an amount of illumination light almost uniform throughout the length of one line of a document reading unit. Reference numeral 7 denotes connector which connects a sensor signal to an external device; and numeral 8 denotes a document support table made of a transparent glass material, which supports the document 9.
The light emitted from the LEDs arranged on the end face of the optical waveguide member 2 is guided through the optical waveguide member 2 made of an acrylic material and exits outside from the light exit surface after being reflected in a complicated manner in the optical waveguide member 2. The light then illuminates the document 9. In order to reflect this light, it is preferable to place a titanium oxide powder or an aluminum powder on the reflecting surface or terminal end portion of the optical waveguide member 2 in advance by using a printing means or the like.
As this linear lighting apparatus, a linear lighting apparatus is used, which has light-emitting elements provided on one end portion of a rod-like transparent member made of an acrylic material with the central position of a reflecting surface being shifted from that of a light source (patent reference 1). In addition, an optical waveguide member having inside two surfaces with reflecting surfaces set at different angles (patent reference 2). The linear lighting apparatuses disclosed in these references are designed to achieve reductions in cost by arranging LEDs, which are light-emitting elements, on only one end of a rod-like transparent member. The shape of an optical waveguide member is also designed to obtain uniform illuminance along the longitudinal direction so as to obtain illuminance which is made as uniform as possible from one end which light from LED elements strikes to the other end.
Patent reference 3 discloses a technique of keeping the illuminance on a document surface almost constant within the depth-of-field range of an image formation means by arranging two light sources and vertically shifting the irradiation positions of the respective light sources on the optical axis of a light-receiving element. In addition, patent reference 4 discloses an optical waveguide member whose central portion is bifurcated into two portions so as to hold a lens array.    Patent reference 1: Japanese Patent No. 2693098    Patent reference 2: Japanese Patent Laid-Open No. 2001-159796 (FIG. 1)    Patent reference 3: Japanese Patent No. 2848477    Patent reference 4: Japanese Patent Laid-Open No. 11-266340